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Re: Preamp acting as a microphone

Posted: Fri Feb 05, 2010 4:25 pm
by Doug Coulter
We tested ours with a few megs -- about 3. The pulses from a proportional mode tube are tiny indeed, and you may have them and just not be seeing them. For grins, we tried running up the volts on our tube to the point where you get "gas gain", which in our case was about 2700 volts, and then we got nice fat pulses - volts, but that's maybe not the greatest idea for having the tube live either.

The amount of energy stored in .1 uf cap at those voltages could itself utterly fry a tube, I'd go waaay down from there. No need for response to near DC.

Re: Preamp acting as a microphone

Posted: Fri Feb 05, 2010 4:53 pm
by Richard Hull
Again, a good coupling cap would be a 500pf or less at 5kv. Some couplers I have seen are on the order of 10pf at 5kv. You are coupling sub-microsecond signals.

Disconnect all BNC's from the tube and supply to the preamp and start tapping. A disconnected HV supply doesn't mean the tube will not still give microphonics.

Elimenate piece by piece

Richard Hull

Re: Preamp acting as a microphone

Posted: Sat Feb 06, 2010 1:39 am
by bpaddock
Steven Sesselmann wrote:

> I moved the two 9 volt batteries that power the preamp, out of the aluminum box, and the microphonics virtually stopped.
What brand are the batteries? We had to switch from Energizer to Duracell's 9V's at work (we use so many Duracell's you may have seen our product in a Duracell commercial).

The Energizers don't have enough compression to keep the stack intact if dropped or moved suddenly. You may have one that is on the edge of opening, which can happen after temperature cycling, so little vibration modulates your power supply.

Re: Preamp acting as a microphone

Posted: Sat Feb 06, 2010 7:09 am
by Steven Sesselmann
Thanks again for all the help, I now have a whole lot more respect for those reliable survey meters that just keep on going on, these charge sensitive preamps are temperamental to say the least. once you get it right, "Dont touch it".

Okay, I moved the two Duracell batteries out of the box, and that helped the microphonics, then I resoldered the caps on the circuit board and changed the two 100MR resistors with one 10 MR resistor.

I still had nothing but noise

I then became suspicious of the dual HV NIM power supply that I got on ebay, other than the red indicator light I had no way of testing if it worked. So I dug out an old antique bench style rate meter and used it as a HV supply for the 3He tube, and there were my pulses

Now I still have some learning to do...

There are a number of random small pulses, interrupted by the odd pulse which hits 2.5V on my scope screen, about every couple of seconds. probably too often to be stray neutrons ... I hope.

The problem is pulses never go higher than 2.5V.

Is this an output limit of the preamp, or is it the input limit of my scope?

Now if I am looking for neutrons, I assume I am looking for something bigger, so how can I figure out which is which?



1) Signal directly from preamp with 3He tube (1400V bias)

2) Signal after it has gone through NIM SCA (single channel analyzer) and NIM amp.

3) The whole mess will look a lot neater when I get it all up in a U rack.

Re: Preamp acting as a microphone

Posted: Sun Feb 07, 2010 12:26 am
by Steven Sesselmann
....Have you guys heard the expression

"When you ass'u'me you make an ass out of 'u' and me...

Well Roman Radke was almost right, I just discovered how silly I have been, spending three evenings trouble shooting this preamp problem..

The dual NIM power supply I got from eBay, has two ten turn knobs at the front, marked A and B and two SHV connectors at the back, marked A and B, pretty simple right...., except when the NIM is in the rack, I can't see the back, so I naturally assume that the plug on the left corresponds with the knob on the same side of the NIM, because I am working from the front leaning over the box...

Not so, whoever designed this thing decided to cross the connections over.

Now I am really embarrassed....


Re: Preamp acting as a microphone

Posted: Sun Feb 07, 2010 4:28 am
by John Futter
I think the correct word is

you will not be doing that again!!!!!!!!!!!!!??

Re: Preamp acting as a microphone

Posted: Sun Feb 07, 2010 12:33 pm
by Starfire
Two steps forward - one step back, Steven

Re: Preamp acting as a microphone

Posted: Mon Feb 08, 2010 5:32 pm
by Richard Hull
A moves naturally to B, left to right as in natural alphabetical reading. This is observed on both the front and the back, thus, the seeming reversal to someone feeling they should go straight through. The important thing is they are marked pefectly correctly. This convention is generally observed on all gear.

Without a hot radium or gamma source next to the tube, you can't be sure. The taller pulses after the SCA windowing are probably all cosmics or ultra hot gammas from radon daughters, but it seems about right. I would crank the upper level detect a bit higher until I saw about 5-10 cpm max if I did not have a hot gamma source at hand. Big tubes would see about 10-15 as background and small tubes less than 5. The little Nancy Woods BF3 tubes in rem balls see about 2cpm background.

Richard Hull

Re: Preamp acting as a microphone

Posted: Sat Apr 20, 2019 8:02 am
by Rich Feldman
Count me in as another hobbyist dealing with microphonic components.

My BF3 tube - charge sensitive preamplifier - oscilloscope benchtop experiment came together this week, as reported in another thread. A major nuisance is trigger events whose displayed waveforms don't look like normal pulses from ionizing-radiation. They happen for sure any time there's a mechanical disturbance.

Before turning to Internet and, I dedicated a short lab session to locating the cause.
Took control of the scope Horizontal and Vertical, while tapping and wiggling things.
The most common erratic waveform is a lightly damped oscillation at about 260 Hz.
Looks like the main culprits are the HV blocking cap (a ceramic disk) and the BF3 tube itself. was more productive than Google, in revealing that both device types are familiar offenders.

NIM bin, bias power supply, home-made SHV connector, and HV coax cable are off the hook. Each can be jiggled without causing a disturbance on the scope screen. And the bias circuit inside preamp box begins with a 47-ms lowpass filter (10 MΩ, 4.7 nF).

Next step: replace the blocking cap with one made of better stuff.
Found a 330 pF, 2000 V silver-mica capacitor in parts bin!
smc.JPG (12.49 KiB) Viewed 4318 times
Practically ideal, I think, while suspending the desire for 3000 V rating on all HV caps. The reduced capacitance will allow HV bias to be ramped up or down 3 times faster than before, with no more stress on the preamp circuit. To use a plastic film capacitor would require shopping, or squeezing in something electrically and physically too big.